The use of multi-user, radio communication systems has achieved wide popularity in recent years. Advancements in communication technologies have permitted the implementation of such radio communication systems to be afforded by a large number of users.
In a radio communication system, a communication channel connects a sending and a receiving station. A communication channel uses a portion of the electromagnetic spectrum to create a wireless connection, replacing wire line connections. Communication by way of a radio communication system provides the particular advantage of untethered communications.
A sending station of a radio communication system converts information which is to be communicated to the receiving station into a signal with characteristics that permit its propagation over the communication channel. To convert the information into a communication signal, the sending station modulates the information onto a carrier wave with an assigned frequency.
Mixer circuits typically form a portion of a sending station. The information to be communicated by the sending station is initially at baseband frequency. The mixer circuit of the sending station up-converts in frequency the baseband information. A sending station sometimes includes more than one mixer circuit. When multiple mixing stages are utilized, an IF (intermediate frequency) signal is formed at the first mixer stage and a RF (radio frequency) signal is formed at the final mixing stage of the sending station.
A typical receiving station receives a radio frequency communication signal by way of a communication channel and analogously converts the radio frequency signal into a baseband signal. Thereafter, the information of the received communication signal is recovered. The receiving station utilizes one or more mixer circuits to down-covert in frequency the received communication signal into a baseband signal. When multiple mixing stages are utilized, an IF signal is formed at the first mixer stage and a baseband signal is formed at the final mixing stage.
The performance of the receiving station is defined, amongst other things, by its sensitivity and selectivity. Sensitivity is the ability of the receiving station to recover information in the presence of noise and depends on various factors, including noise figure, information bandwidth, and carrier-to-noise ratio requirements. The noise figure is set by the front-end down converter of the receiving station. Selectivity is the ability of the receiving station to isolate information amongst interfering signals. The selectivity of the receiving station is determined by channel filtering and signal handling characteristics.
The mixer circuit plays a big part in determining the performance of the receiving station. A typical front-end down converter of the receiving station includes a low noise amplifier, an image rejection filter, and a mixer circuit. A conventional mixer circuit is generally categorized to be either a passive circuit or an active circuit.
Passive mixer circuits generally exhibit a low noise figure and are operable over a wide dynamic range. However, passive mixer circuits generally require relatively high local oscillator drive and low noise IF (intermediate frequency) amplification to compensate for relatively high insertion loss. When power considerations are significant, such as when the mixer circuit is to form a portion of a low power integrated circuit, powered by a portable power supply, the relatively high power requirements associated with passive mixer circuits are disadvantageous.
Conversely, active mixer circuits exhibit higher noise figures and operate over narrower dynamic ranges. However, active mixer circuits use less power because they require less IF amplification and lower local oscillator drive. The reduced power requirements of active mixer circuits are generally preferred in applications related to low-power, integrated electronics. These active mixer circuits are especially advantageous for mobile stations used in radio communication systems, such as cellular communication systems. However, for these requirements, standard active mixer circuits do not offer high enough performance and improved active mixer circuits are needed.
An active mixer exhibiting improved dynamic range and ease of design would be advantageous. Such an active mixer could form a portion of a mobile station operable in a cellular, or other radio, communication system.
It is in light of this background information related to mixer circuits that the significant improvements of the present invention have evolved.